Metal-coated articles and method of making



United States, Patent METAL-COATED ARTICLES AND METHOD on MAKING I Lowell D. Eubank, Richland, Wash., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application April 16, 1945 Serial No. 588,708

' 'Claims, (c1. 29-194 This invention relates to the application of metal coatings to uranium and to the resulting coated uranium products. Such products are useful in a neutronic reactor of the type described in the c-o-pending application of Fermi, et al., S. N. 568,904 filed December 19, 1944, now Patent No. 2,708,656, issued May 17, 1955 Metallic uranium is subject to rapid attack upon exposure to air, water or chemicals. In the past it' has been proposed to protect the uranium by means of metal coatings such as zinc coatings. Zinc alonesuifers from the defects that its adherence to the metallic uranium is poor at high temperatures and the uranium tends to a diffuse through the zinc and contaminate the coating exposure to severely corrosive conditions the coatings tend to deteriorate.

Lead constitutes a valuable protective, coating material but unites with uranium to form a pyrophoric alloy. The application of lead over zinc-aluminum alloys produces inferior coatings which separate readily at the ,r'nargin of the lead and zinc-aluminum alloy.

It is an object of the present invention to provide more satisfactory coatings for metallic uranium than heretofore known. A further object is the provision of coatings especially useful for protecting the uranium at high temperatures such as 250 C. to 300 C. A still further object is the provision of adherent protective coatings comprising lead. Further objects will appear fromthe following general description and specific illustrations of the invention.

According to the present invention metallic uranium is first dipped into a molten metal bath of 'zinc and is then dipped into a second molten metal bath of lead containing a minor proportion of silver. 7

The zinc bath may consist of a high grade zinc free from substantial quantities of lead or iron, which seem to be deleterious to the continuity of zinc coatings on uranium. However, it is preferable to employ a zinc bath containing a minor proportion of aluminum, in which case less pure grades of zinc may be employed to form continuous coatings and the coatings obtained are more durable at high temperatures.

The preferred treatment results in the formation of a multiple layer protective coating consisting of a layer of uranium-aluminum alloy directly uponthe metallic uranium, a layer of zinc containing a minor proportion.

of aluminum upon theuranium-aluminum alloy layer,

1 21,848,797} Patented Aug. 26, 1958 ICC layer so as to provide an integral protective coating firmly' adherent to the metallic uranium article to be protected.

The uranium-aluminum alloy and aluminum-zinc alloy layers accomplish several purposes. In addition to providing a very considerable degree of protection to the coated metal, they serve as bonding media for improving the adherence of the overlying lead alloy coatings. Furthermore, they serve to inhibit the diffusion of uranium into the lead coatings, and in this way prevent the formation of a pyrophoric uranium-lead alloy.

Each of the molten metal baths employed in applying the coatings of the present invention may be varied over a wide range of proportions. Thus the zinc-aluminum bath may contain as little as 1% or as much as 45% or more by weight of aluminum and preferably contains between about 5% andabout 15% by weight of aluminum and between about 95% andabout by applied. Thus higher temperatures are-*most suitable for baths of a composition relativelyremote from the eutectic alloy composition zinc, 5% aluminum) and lower temperatures are most suitable for alloys nearer to this composition. I

The lead bath is preferably maintained at a substantially lower temperature and most suitably at a temperature between the melting point of the lead bath and the melting point of the zinc bath. Forapplication of lead coatings to uranium previously dipped'in an 85% zinc-15% aluminum bath, lead bath temperatures between 350 and 420 C. have been found to be satisfactory and about 400 C. is preferred. 7

The lead coating may be applied either before or after the zinc coating has solidified and in the latter case either before or after the metal has been allowed to coolto ordinary temperatures. j

The surface of the metallic uranium should be suitably prepared for application of the metal coatings by an appropriate treatment for removing grease and dirt and metal oxides. A preliminary treatment in nitric acid has been found effective forre'moving metal oxide film from the metallic uranium surface. A sand-blast cleaning or an .anodic treatment in sulfuric acid also may be used for this purpose. An especially satisfactory preparatory treatment involves dipping the metal in aqueous 50% to 70% nitric acid solution for between. 4 and ,6 minutes at a temperature between 60 and 70 C. This treatment is the subject of U. S. patent application Serial Number 619,265 filed September 28, 1945, by Ernest R.

Boller, Lowell D. Eubank and John W. Robinson eninsure adequate coverage.

D. Eubank and entitled Metal Surface Treatment. It is preferred to employ the flux just described as a protective flux on the zinc bath. The lead bath may be provided with a fiux such as a eutectic mixture of zinc and ammonium chlorides. It is preferred, however, to employ a dry lead bath. The molten lead bath frees the metal surface ofvfiux, so that deposits of solid flux on the coating during cooling are eliminated. 1

After the metallic uranium has been coated, it may be cooled in air to atmospheric temperature or, especially when the zinc bath contains relatively high proportions of aluminum, it may be cooled in an annealing oven maintained at a temperature between 200 and 300 C. to provide very slow cooling through the high temperature range. After the coatings have cooled to a temperature of about 250 to 275 0., they may be cooled as rapidly as desired, for example, by .quenching in water or other suitable quenching media.

For application of uniform continuous coatings to cylindrical objects the coated objects may be rolled upon smooth continuous surfaces as described, for example, in U. S. patent application Serial Number 577,382 filed February 12, 1945, by Harold A. Gage and entitled Method of Leveling Metal Coatings.

The following examples, in which proportions are expressed in terms of weight, will further illustrate the .invention.

Example 1 An extruded uranium rod about 1.1 inches in diameter and 4 inches in length was dipped for 3 minutes in an aqueous 50% HNO solution maintained at a temperaand 15% aluminum maintained at a temperature of After the rod had remained in this bath for 4 minutes, it was withdrawn, shaken briskly to remove excess molten metal and dipped immediately into a dry molten metal bath comprising 98% lead and 2% silver maintained at a temperature of 400 C. It was promptly withdrawn from this bath and dipped twice more into the same bath to Upon removal from this bath for the third time, it was placed on smooth Transite (asbestos cement composition) rollers and rolled until it had solidified. It was then allowed to cool in air by normal radiation.

To determine the durability of the coating under adverse conditions the rod was placed in a current of air at 200 C. After 44 days of exposure the coating was still in perfect condition. The rod was then placed in a current of air maintained at 250 C. where it remained for 14 days more. At the end of this testing period the coating was still in perfect condition.

Example 2 An extruded uranium rod of the type employed in Example 1 was dipped into an aqueous 60% nitric acid solution at 65 C. for minutes, then removed, rinsed and dried.

It was then dipped through a layer of flux of the same composition as in Example 1 into an 85% zinc-15% aluminum bath followed by a single dip in a 98% lead-2% silver bath. The period of immersion in the zinc-aluminum bath was 3 minutes and the temperature was 550 C. In the lead-silver bath the period of immersion was 1 /2 minutes and the temperature was 425 C. The product was rolled as in Example 1 until the coating had solidified,

, and it was then cooled in air to room temperature.

base metal.

4 raised to 250 C. The coating was still in perfect condition whenremoved from the test 9.0 days later.

Example 3 An extruded uranium rod inch in diameter and 2 inches in length was cleaned by dipping for 5 minutes in 50% nitric acid solution at C. and for 20 seconds re in an a u qus so ut o consisting f e u P r by volume of 44% lactic acid and 78% (60 B.) sulfuric acid at 82 C.

It was then dipped through a flux comprising .42 parts lithium chloride,5 3 parts potassium chloride and 5 parts sodium chloride into a zinc-aluminum bath containing zinc and 15% aluminum at a temperature of 530 C. After the rod had remained in this bath for 4 minutes, it was removed and rolled in air on Transite rollers for 30 seconds.

It was then dipped through a flux comprising 73% zinc hloride and 27% ammonium chloride into a molten lead silver bath containing 98% lead and 2% silver at 380 The rod was allowed to remain in this bath for .45 seconds and was then removed, rolled as before until the eoating had solidified and then allowed to cool natural y i a This eoating was tested by placing the coated rod in a current pf air at 250 C. After 207 days of exposure theen de e d s ernible wa single pinhole n h seatin E amp 4 a flux containing 37% lithium chloride, 53% potassium chloride and 10% sodium chloride into a zinc-aluminum bath containing 85% zinc and 15 aluminum at 550 C.

After 40 seconds in this bath, the rod was withdrawn and quenched by contact with cold water.

' It was then-dipped through a flux comprising the eutectic mixture of ammonium and zinc chlorides into a leadsilver bath containing 2% of silver at 365 C. After 1 minute in this bath, the rod was withdrawn and quenched as before.

The surface was rough and exhibited pinholes extending part way through the coating but none exposing the Despite the apparent defectiveness of this coating, it completely protected the metallic uranium upon exposure for 48 days to a current of air at 200 C.

Example 5 An extruded metallic uranium rod inch in diameter by 2 inches in length was cleaned in aqueous 50% nitric acid solution and wiped dry.

It was then dipped through a molten flux comprising 37% LiCl, 53% KCl and 10% NaCl into special high grade zinc (ASTM Specification 1A) at 560 C. After 30 seconds in this bath the rod was removed, cooled in air for 10 seconds and rolled on smooth Transite rollers until the coating solidified, quenched in water and wiped dry.

The coated rod was dipped through a flux comprising 73% ZnCl and 27% NH Cl into a bath consisting of 98 parts of lead and 2 parts of silver at 340 C. After 30 seconds in this bath, the rod was withdrawn, rolled on Transite rollers and water-quenched.

A smooth, continuous coating on the rod was thus secured comprising a layer of lead-silver alloy about 2.4 mils thick over a zinc layer about 6.7 mils thick.

It will be understood that I intend to include variations and modifications of the invention and that the preceding examples are illustrations only and in no wise to be construed as limitations upon the invention, the scope of which is defined in the appended claims, wherein I claim:

1. As a new article of manufacture a solid metallic uranium body having thereon an integral multiple layer protective coating comprising a continuous adherent zinc under-layer and a lead-silver overlayer firmly bonded to the zinc under-layer by a silver-zinc alloy layer.

2. As a new article of manufacture a solid metallic uranium body having thereon an integral multiple layer protective coating comprising an inner uranium-aluminum alloy layer firmly bonded to the metallic uranium, 'a second layer of zinc-aluminum alloy firmly bonded to the uranium-aluminum alloy layer, a third layer of silverzinc alloy firmly bonded to the zinc-aluminum alloy layer,

. and a fourth layer of lead-silver alloy firmly bonded to the silver-zinc alloy layer.

3. As a new article of manufacture a solid metallic uranium body having thereon an integral multiple layer protective coating comprising an inner uranium-aluminum alloy layer firmly bonded to the metallic uranium, a second layer of zinc containing a minor proportion of aluminum firmly bonded to the uranium-aluminum alloy layer, a third layer of silver-zinc alloy firmly bonded to the zinc layer, and a fourth layer of lead containing a minor proportion of silver firmly bonded to the silverzinc alloy layer.

4. As a new article of manufacture a solid metallic uranium body having thereon an integral multiple layer protective coating comprising an inner uranium-aluminum alloy layer firmly bonded to the metallic uranium, a second layer of zinc containing about 5% to about 15% of aluminum firmly bonded to the uranium-aluminum alloy layer, a third layer of silver-zinc alloy firmly bonded to the zinc layer, and a fourth layer of lead containing about 0.1% to about 5% of silver firmly bonded to the silver-zinc alloy layer.

5. The method of protectively coating a metallic uranium article, which comprises dipping the metallic uranium article through an alkali-metal halide protective flux into a molten metal bath containing between about 85% and about 95% zinc and between about 5% and about 15% aluminum, withdrawing the metallic uranium article from this coating bath and dipping it into a second molten metal bath containing between about 95% and about 99% lead and between about 0.1% and about 5% silver, withdrawing the metallic uranium article from the second bath and causing the coating thereon to cool and solidify.

6. The method of protectively coating a metallic uranium article, which comprises dipping the article through an alkali-metal halide flux into a molten metal bath containing a major proportion of zinc and a minor proportion of aluminum at a temperature between 20 and 200 C. above the melting point of the metal bath, withdrawing the article from said bath and, while the coating metal thereon is still molten, dipping the article into a dry molten metal bath comprising a major proportion of lead and a minor proportion of silver at a lower temperature, withdrawing the article from said second bath and causing the coating thereon to cool and solidify.

, 7. The method of protectively coating a metallic uranium article, which comprises dipping the article through a molten alkali-metal halide flux into a molten metal bath containing between about 85% and about 95 of zinc and between about 5% and about 15% of aluminum at a temperature between 20 and 200 C. above the melting point of the metal bath, withdrawingthe article from said bath and, while the metal coating thereon is still molten, dipping it into a dry molten metal bath comprising between about 95% and about 99% lead ,and between about 0.1% and about 5% of silver at a temperature between the melting points of said two molten metal baths, withdrawing the article from said dry bath and cooling it gradually to a temperature between about 250 and about 275 C. to solidify and anneal the coating thereon.

8. The method of protectively coating a metallic uranium article, which comprises dipping the article through an alkali-metal halide protective flux into a molten metal bath containing a major proportion of zinc and a minor proportion of aluminum, withdrawing the article from said bath and cooling it .to cause the coating thereon to solidify, dipping the coated article through a zinc-aluminum chloride flux into a molten metal bath comprising a major proportion of lead and a minor proportion of silver, withdrawing the article from said second bath and causing the metal coating thereon to cool and solidify.

9. The method of protectively coating a metallic uranium article, which comprises dipping the article through an alkali-metal halide flux into a molten metal bath containing a major proportion of zinc and a minor proportion of aluminum at a temperature between 20 and 200 C. above the melting point of the metal bath, withdrawing the article from said bath and cooling it' to cause the coating thereon to solidify, dipping the coated article through a zinc-aluminum chloride flux into a molten metal bath comprising a major proportion of lead and a minor proportion of silver at a lower temperature, withdrawing the article from said second bath and causing the coating thereon to cool and solidify.

10. The method of protectively coating a metallic uranium article, which comprises dipping the article through a molten alkali-metal halide flux into a first molten metal bath containing between about andabout of zinc and between about 5% and about 15% of aluminum at a temperature between 20 and 200 C. above the melting point of the metal bath, withdrawing the article from said bath and cooling it to cause the coating thereon to solidify, dipping the coated article through a zinc-aluminum chloride flux into a molten metal bath comprising between about 95% and about 99% lead and between about 0.1% and about 5% of silver at a temperature between the melting points of said two molten metal baths, withdrawing the article from said second bath and cooling it gradually to a temperature between about 250 and about 275 C. to solidify and anneal the coating thereon.

References Cited in the file of this patent UNITED STATES PATENTS 943,161 Rockey et a1. Dec. 14, 1909 1,378,052 Peacock May 17, 1921 1,722,949 Wehr et al. July 30, 1929 1,943,853 Austin -r- Ian. 16, 1934 FOREIGN PATENTS 7,355 Great Britain of 1837 

1. AS A NEW ARTICLE OF MANUFACTURE A SOLID METALLIC URANIUM BODY HAVING THERON AN INTEGRAL MULTIPLE LAYER PROTECTIVE COATING COMPRISING A CONTINUOUS ADHERENT ZINC UNDER-LAYER AND LEAD-SILVER OVERLAYER FIRMLY BONDED TO THE ZINC UNDER-LAYER BY A SILVER-ZINC ALLOY LAYER.
 5. THE METHOD OF PROTECTIVELY COATING A METALLIC URANIUM ARTICLE, WHICH COMPRISES DIPPING THE METALLIC URANIUM ARTICLE THROUGH AN ALKALI-METAL HALIDE PROTECTIVE FLUX INTO A MOLTEN METAL BATH CONTAINING BETWEEN ABOUT 85% AND ABOUT 95% ZINC AND BETWEEN ABOUT 5% AND ABOUT 15% ALUMINUM, WITHDRAWING THE METALLIC URANIUM ARTICLE FROM THIS COATING BATH AND DIPPING IT INTO A SECOND MOLTEN METAL BATH CONTAINING BETWEEN ABOUT 95% AND ABOUT 99% LEAD AND BETWEEN ABOUT 0.1% AND ABOUT 5% SILVER, WITHDRAWING THE METALLIC URANIUM ARTICLE FROM THE SECOND BATH AND CAUSING THE COATING THEREON TO COOL AND SOLIDIFY. 